The present invention relates to a magnetically anisotropic magnetic powder composed of a rare earth element-iron-boron-gallium alloy powder, and a permanent magnet composed of such alloy powder dispersed in a resin, and more particularly to a resin-bonded permanent magnet having good thermal stability composed of a magnetically anisotropic rare earth element-iron-boron-gallium permanent magnet powder having fine crystal grains dispersed in a resin.
Typical conventional rare earth element permanent magnets are SmCo.sub.5 permanent magnets, and Sm.sub.2 Co.sub.17 permanent magnets. These samarium cobalt magnets are prepared from ingots produced by melting samarium and cobalt in vacuum or in an inert gas atmosphere. These ingots are pulverized and the resulting powders are pressed in a magnetic field to form green bodies which are in turn sintered and heat-treated to provide permanent magnets.
The samarium.cobalt magnets are given magnetic anisotropy by pressing in a magnetic field as mentioned above. The magnetic anisotropy greatly increases the magnetic properties of the magnets. On the other hand, magnetically anisotropic, resin-bonded samarium.cobalt permanent magnets are obtained by injection-molding a mixture of samarium.cobalt magnet powder produced from the sintered magnet provided with anisotropy and a resin in a magnetic field, or by compression-molding the above mixture in a die.
Thus, resin-bonded samarium cobalt magnets can be obtained by preparing the sintered magnets having anisotropy, pulverizing them and then mixing them with resins as binders.
Recently, neodymium-iron-boron magnets have been proposed as new rare earth magnets surmounting the samarium.cobalt magnets containing samarium which is not only expensive but also unstable in its supply. Japanese Patent Laid-Open Nos. 59-46008 and 59-64733 disclose permanent magnets obtained by forming ingots of neodymium-iron-boron alloys, pulverizing them to fine powders, pressing them in a magnetic field to provide green bodies which are sintered and then heat-treated, like the samarium.cobalt magnets. This production method is called a powder metallurgy method. Also, it was reported to obtain a resin-bonded magnet having magnetic anisotropy by pulverizing an ingot to 0.5-2 .mu.m and then solidifying it with a wax (Appl. Phys. Lett. 48 (10), Mar. 1986, pp.670-672).
With respect to the Nd-Fe-B permanent magnet, an alternative method has been proposed to the above-mentioned powder metallurgy method.
This method comprises melting a mixture of neodymium, iron and boron, rapidly quenching the melt by such a technique as melt spinning to provide fine flakes of the amorphous alloy, and heat-treating the flaky amorphous alloy to generate an Nd.sub.2 Fe.sub.14 B intermetallic compound. The fine flakes of this rapidly-quenched alloy is solidified with a resin binder (Japanese Patent Laid-Open No. 59-211549). However, the magnetic alloy thus prepared is magnetically isotropic. Then, Japanese Patent Laid-Open No. 60-100402 discloses a technique of hot-pressing this isotropic magnetic alloy, and then applying high temperatures and high pressure thereto so that plastic flow takes place partially in the alloy thereby imparting magnetic anisotropy thereto.
The conventional Nd-Fe-B permanent magnets, however, have the following problems.
First, although the above powder metallurgy can provide magnetic anisotropy and magnetic properties of (BH)max=35-45MGOe, the resulting magnets essentially have low Curie temperature, large crystal grain size and poor thermal stability. Accordingly, they cannot be suitably used for motors, etc. which are likely to be used in a high-temperature environment.
Second, although molding is relatively easy by compression molding if rapidly-quenched powder is mixed with a resin, the resulting alloy is isotropic, so that its magnetic properties are inevitably low. For instance, the magnetic properties are (BH)max of 3-5MGOe for those obtained by injection molding and (BH)max of 8-10MGOe for those obtained by compression molding, and further the magnetic properties vary widely depending upon the strength of a magnetic field for magnetizing the alloy. To achieve (BH)max of 8MGOe, the magnetic field should be 50 kOe or so, and it is difficult to magnetize the alloy after assembling for various applications.
In addition, although hot pressing of the rapidly-quenched alloy powder serves to increase the density of the alloy, eliminating pores from the pressed alloy powder to improve weathering properties thereof, the resulting alloy is isotopic so that it is disadvantageous just like the permanent magnet prepared by mixing rapidly-quenched alloy powder with a resin. (BH)max of the resulting alloy is improved in proportion to the increase in the density, and it can reach 12MGOe or so. However, it is still impossible to magnetize it after assembling.
By the method of hot-pressing rapidly-quenched alloy powder and then causing plastic flow therein, anisotropy can be achieved like the powder metallurgy method, providing (BH)max of 34-40MGOe, but annular magnets, for instance, magnet rings of 30 mm in outer diameter, 25 mm in inner diameter and 20 mm in thickness cannot easily be formed because die upsetting should be utilized to provide anisotropy.
Finally, with respect to magnets prepared by pulverizing ingots and solidifying them with wax, powders used are so fine that they are likely to be burned, making it impossible to handle them in the atmosphere. Also since the magnets show a low squareness ratio in the magnetization curve, they cannot have high magnetic properties.
Incidentally, attempts were made to provide anisotropic resin-bonded magnets by pulverizing anisotropic sintered magnets prepared by the powder metallurgy method, mixing the pulverized particles with resins and molding them while applying a DC magnetic field, but sufficiently high magnetic properties could not be achieved.